Abstract
Basalts are recognized as one of the major habitats on Earth, harboring diverse and active microbial populations. Inconsistently, this living component is rarely considered in engineering ...operations carried out in these environments. This includes carbon capture and storage (CCS) technologies that seek to offset anthropogenic CO
2
emissions into the atmosphere by burying this greenhouse gas in the subsurface. Here, we show that deep ecosystems respond quickly to field operations associated with CO
2
injections based on a microbiological survey of a basaltic CCS site. Acidic CO
2
-charged groundwater results in a marked decrease (by ~ 2.5–4) in microbial richness despite observable blooms of lithoautotrophic iron-oxidizing Betaproteobacteria and degraders of aromatic compounds, which hence impact the aquifer redox state and the carbon fate. Host-basalt dissolution releases nutrients and energy sources, which sustain the growth of autotrophic and heterotrophic species whose activities may have consequences on mineral storage.
Carbon capture and storage (CCS) provides a solution toward decarbonization of the global economy. The success of this solution depends on the ability to safely and permanently store CO₂. This study ...demonstrates for the first time the permanent disposal of CO₂ as environmentally benign carbonate minerals in basaltic rocks. We find that over 95% of the CO₂ injected into the CarbFix site in Iceland was mineralized to carbonate minerals in less than 2 years. The result contrasts with the common view that the immobilization of CO₂ as carbonate minerals within geologic reservoirs takes several hundreds to thousands of years. Our results, therefore, demonstrate that the safe long-term storage of anthropogenic CO₂ emissions through mineralization can be far faster than previously postulated.
•Results from injection of CO2-H2S gas mixture at the CarbFix2 site, SW-Iceland.•Soluble gas mixture was captured in water and injected into hot basaltic reservoir.•The injected CO2 and H2S was ...mineralized within months providing safe and long term storage of the gases.•The cost of the integrated method is US $25/ton of the gas mixture at the CarbFix2 site.
One of the main challenges of worldwide carbon capture and storage (CCS) efforts is its cost. As much as 90% of this cost stems from the capture of pure or nearly pure CO2 from exhaust streams. This cost can be lowered by capturing gas mixtures rather than pure CO2. Here we present a novel integrated carbon capture and storage technology, installed at the CarbFix2 storage site at Hellisheiði, Iceland that lowers considerably the cost and energy required at this site. The CarbFix2 site, located in deeper and hotter rocks than the original CarbFix site, permits the continuous injection of larger quantities of CO2 and H2S than the original site. The integrated process consists of soluble gas mixture capture in water followed by the direct injection of the resulting CO2-H2S-charged water into basaltic rock, where much of the dissolved carbon and sulfur are mineralized within months. This integrated method provides the safe, long-term storage of carbon dioxide and other acid gases at a cost of US $25/ton of the gas mixture at the CarbFix2 site and might provide the technology for lower CCS cost at other sites.
Results from injection of 175 tonnes of CO2 into the basaltic subsurface rocks at the CarbFix site in SW-Iceland in 2012 show almost complete mineralisation of the injected carbon in less than two ...years (Matter et al., 2016; Snæbjörnsdóttir et al., 2017). Reaction path modelling was performed to illuminate the rate and extent of CO2-water-rock reactions during and after the injection. The modelling calculations were constrained by the compositions of fluids sampled prior to, during, and after the injection, as reported by Alfredsson et al. (2013) and Snæbjörnsdóttir et al. (2017). The pH of the injected fluid, prior to CO2 dissolution was ∼9.5, whereas the pH of the background waters in the first monitoring well prior to the injections was ∼9.4. The pH of the sampled fluids used in the modelling ranged from ∼3.7 at the injection well to as high as 8.2 in the first monitoring well. Modelling results suggest that CO2-rich water-basalt interaction is dominated by crystalline basalt dissolution along a faster, high permeability flow path, but by basaltic glass dissolution along a slower, pervasive flow path through which the bulk of the injected fluid flows. Dissolution of pre-existing calcite at the onset of the injection does not have a net effect on the carbonation, but does contribute to a rapid early pH rise during the injection, and influences which carbonate minerals precipitate. At low pH, Mg, and Fe are preferentially released from crystalline basalts due to the higher dissolution rates of olivine, and to lesser extent pyroxene, compared to plagioclase and glass (Gudbrandsson et al., 2011). This favours the formation of siderite and Fe-Mg carbonates over calcite during early mineralisation. The model suggests the formation of the following carbonate mineral sequences: siderite at pH < 5, Mg-Fe-carbonates and Ca-Mg-Fe-carbonates at pH > 5, and calcite at higher pH. Other minerals forming with the carbonates are Al- and Fe-hydroxides and chalcedony, and zeolites and smectites at elevated pH. The most efficient carbonate formation is when the pH is high enough for formation of carbonates, but not so high that zeolites and smectites start to form, which compete with carbonates over both cations and pore space. The results of reaction path modelling at the CarbFix site in SW-Iceland indicate that this “sweet spot” for mineralisation of CO2 is at pH from ∼5.2 to 6.5 in basalts at low temperature (20–50 °C).
Abstract
The engineered removal of atmospheric CO
2
is now considered a key component of mitigating climate warming below 1.5 °C. Mineral carbonation is a potential negative emissions technique that, ...in the case of Iceland’s CarbFix experiment, precipitates dissolved CO
2
as carbonate minerals in basaltic groundwater settings. Here we use calcium (Ca) isotopes in both pre- and post-CO
2
injection waters to quantify the amount of carbonate precipitated, and hence CO
2
stored. Ca isotope ratios rapidly increase with the pH and calcite saturation state, indicating calcite precipitation. Calculations suggest that up to 93% of dissolved Ca is removed into calcite during certain phases of injection. In total, our results suggest that 165 ± 8.3 t CO
2
were precipitated into calcite, an overall carbon storage efficiency of 72 ± 5%. The success of this approach opens the potential for quantification of similar mineral carbonation efforts where drawdown rates cannot be estimated by other means.
•CO2 and H2S continue to be injected into subsurface basalts after 3 years.•Over 60% of the injected CO2 is still mineralized within 4 months of injection.•Over 85% of the injected H2S is still ...mineralized within 4 months of injection.•Increased greenhouse gas injection rates accelerated mineralization rates.
The CarbFix method was upscaled at the Hellisheiði geothermal power plant to inject and mineralize the plant’s CO2 and H2S emissions in June 2014. This approach first captures the gases by their dissolution in water, and the resulting gas-charged water is injected into subsurface basalts. The dissolved CO2 and H2S then react with the basaltic rocks liberating divalent cations, Ca2+, Mg2+, and Fe2+, increasing the fluid pH, and precipitating stable carbonate and sulfide minerals. By the end of 2017, 23,200 metric tons of CO2 and 11,800 metric tons of H2S had been injected to a depth of 750 m into fractured, hydrothermally altered basalts at >250 °C. The in situ fluid composition, as well as saturation indices and predominance diagrams of relevant secondary minerals at the injection and monitoring wells, indicate that sulfide precipitation is not limited by the availability of Fe or by the consumption of Fe by other secondary minerals; Ca release from the reservoir rocks to the fluid phase, however, is potentially the limiting factor for calcite precipitation, although dolomite and thus aqueous Mg may also play a role in the mineralization of the injected carbon.
During the first phase of the CarbFix2 injection (June 2014 to July 2016) over 50% of injected carbon and 76% of sulfur mineralized within four to nine months, but these percentages increased four months after the amount of injected gas was doubled during the second phase of CarbFix2 (July 2016 – December 2017) at over 60% of carbon and over 85% of sulfur. The doubling of the gas injection rate decreased the pH of the injection water liberating more cations for gas mineralization. Notably, the injectivity of the injection well has remained stable throughout the study period confirming that the host rock permeability has been essentially unaffected by 3.5 years of mineralization reactions. Lastly, although the mineralization reactions are accelerated by the high temperatures (>250 °C), this is the upper temperature limit for carbon storage via the mineral carbonation of basalts as higher temperatures leads to potential decarbonation reactions.
Injection of CO2 into basaltic formations provides significant benefits including permanent storage by mineralisation and large storage volume. The largest geological storage potential lies offshore ...and in the case of basalt, along the mid-oceanic ridges where CO2 could be stored as carbonate minerals for thousands of years. Most of the bedrock, both on land and offshore Iceland consists of basalt that could theoretically be used for injection of CO2, fully dissolved in water. The most feasible formations are the youngest formations located within the active rift zone. It is estimated that up to 7000 GtCO2 could be stored offshore Iceland within the Exclusive Economic Zone. Site specific geological research and pilot studies are required for refining the concept and offshore pilot scale projects should be considered as the next steps in evolving the method.
The engineered removal of atmospheric CO
is now considered a key component of mitigating climate warming below 1.5 °C. Mineral carbonation is a potential negative emissions technique that, in the ...case of Iceland's CarbFix experiment, precipitates dissolved CO
as carbonate minerals in basaltic groundwater settings. Here we use calcium (Ca) isotopes in both pre- and post-CO
injection waters to quantify the amount of carbonate precipitated, and hence CO
stored. Ca isotope ratios rapidly increase with the pH and calcite saturation state, indicating calcite precipitation. Calculations suggest that up to 93% of dissolved Ca is removed into calcite during certain phases of injection. In total, our results suggest that 165 ± 8.3 t CO
were precipitated into calcite, an overall carbon storage efficiency of 72 ± 5%. The success of this approach opens the potential for quantification of similar mineral carbonation efforts where drawdown rates cannot be estimated by other means.
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